Extinction ratio control of a semiconductor laser

ABSTRACT

The present invention provides a method and apparatus, such as an integrated circuit, to control the extinction ratio of a semiconductor laser and maintain the extinction ratio substantially constant at a predetermined level. Apparatus embodiments include a semiconductor laser, a modulator, a photodetector, and an extinction ratio controller. The semiconductor laser is capable of transmitting an optical signal in response to a modulation current. The modulator is capable of providing the modulation current to the semiconductor laser, with the modulation current corresponding to an input data signal. The photodetector, which is optically coupled to the semiconductor laser, is capable of converting the optical signal into a photodetector current. The extinction ratio controller, in response to the photodetector current, is capable of adjusting the modulation current provided by the modulator to the semiconductor laser to generate the optical signal having substantially a predetermined extinction ratio.

CROSS-REFERENCE TO A RELATED APPLICATION

This application is related to a co-pending application of Jonathan H.Fischer and James P. Howley, U.S. patent application Ser. No. ______,entitled “Optical Midpoint Power Control and Extinction Ratio Control ofa Semiconductor Laser”, filed concurrently herewith, commonly assignedto Agere Systems, Inc., and incorporated by reference herein, withpriority claimed for all commonly disclosed subject matter (the “relatedapplication”).

FIELD OF THE INVENTION

The present invention relates, in general, to semiconductor lasersystems and, more particularly, to an apparatus and method forcontrolling an extinction ratio of a semiconductor laser.

BACKGROUND OF THE INVENTION

Semiconductor lasers, such as laser diodes, produce coherent light wavesat a wide variety of wavelengths of the electromagnetic spectrum, andare ideal for many applications, such as optical fiber communication andnetwork systems, laser printers, bar code scanners, compact discplayers, etc. Laser diodes and other laser semiconductors generallyinclude an active layer or region, using materials such as, for example,gallium arsenide, gallium indium arsenide, gallium nitride, variationsof these materials including zinc oxide and/or phosphorous, or anotherdirect bandgap semiconductor material. Because laser diodes also can bedirectly modulated at high frequencies (e.g., at several GHz), throughmodulation of their “drive current”, laser diodes are often used in highspeed communication and other networking applications, such as for datatransmission, voice communication, multimedia applications, and so on.

Laser performance, including the optical output power provided by thelaser, however, varies due to a number of factors including, forexample, the laser temperature, the age of the semiconductor laser, andprocess variations in semiconductor laser fabrication. Such performancevariations include a change in the laser transfer function, namely, achange in laser output power (“L”) for a given laser diode current (“I”)(forward bias current). These changes in output power and forward biascurrent characteristics may be represented graphically as: (1) a shiftin a graphical slope of the transfer function (laser output power versusforward bias current (L-I slope)) characterizing the semiconductorlaser; and (2) a shift in the laser threshold current (i.e., the levelof forward bias current at which the semiconductor laser firstdemonstrates coherent radiation).

As a consequence, such laser performance variations also causeundesirable variations in an “extinction ratio”, which is defined as theratio of the optical output power resulting from transmission by thesemiconductor laser of a signal representing a data logical 1 (or high),at a first (or comparatively higher) power level, to the optical outputpower resulting from transmission by the semiconductor laser of a signalrepresenting a data logical 0 (or low), at a second (or comparativelylower) power level (i.e., the ratio of the first power level for alogical one to the second power level for a logical zero). Laserperformance variations which diminish the extinction ratio, as aconsequence, cause undesirable variations in the signal-to-noise ratio(SNR) of the system. For example, as the L-I slope characterizing thesemiconductor laser decreases for given forward bias and modulationcurrents, the difference between optical 1 (high) signal power and theoptical 0 (low) signal power decreases, degrading the signal-to-noiseratio.

One aspect of maintaining appropriate signal-to-noise ratios, in lightof laser performance variations, may be achieved by maintaining asubstantially or significantly constant extinction ratio. Prior artattempts to maintain such a constant extinction ratio include use of anopen loop adjustment to a laser diode's operating points, based on thelaser die temperature and a look-up table. Unfortunately, such an openloop system provided no guarantee that the corresponding adjustments tothe operating points were yielding the desired effect of maintaining aconstant extinction ratio, and did not account for the effects of laseraging. In addition, in the open loop system, accurate characterizationof the laser was required to program the look-up table, processvariations were unaccounted for unless each system was programmedindependently, and the laser driver's die temperature may not accuratelyrepresent the semiconductor laser's operating temperature.

Another prior art attempt to maintain a constant extinction ratiosuperimposed a low frequency modulation to the transmit power levelassociated with an optical 1, and then using resulting data, calculatedthe laser diode's L-I slope to approximate the extinction ratio andprovide some form of correction. See Bosch et al. U.S. Pat. No.5,373,387, “Method for Controlling the Amplitude of an Optical Signal”,issued Dec. 13, 1994; and Bosch et al. U.S. Pat. No. 5,448,629,“Amplitude Detection Scheme for Optical Transmitter Control”, issuedSep. 5, 1995. Unfortunately, this extinction ratio determination methodwas inaccurate, as correction to the extinction ratio was sensitive tonon-linearity (curvature) in the semiconductor laser's transfer functioncharacteristics, and the method did not directly determine the actualextinction ratio.

As a consequence, a need remains for an apparatus and method to controlthe extinction ratio of a semiconductor laser.

SUMMARY OF THE INVENTION

The present invention provides an apparatus and method for asemiconductor laser to generate an optical signal having a substantiallyconstant, predetermined extinction ratio. The various apparatusembodiments, such as circuit embodiments, include a semiconductor laser,a modulator, a photodetector, and an extinction ratio controller (havinga sampler and a modulation current controller).

In the various embodiments, the semiconductor laser, such as a laserdiode, is capable of transmitting an optical signal having a firstoptical power level in response to a first modulation current level andhaving a second optical power level in response to a second modulationcurrent level, with the first optical power level being greater than thesecond optical power level. The modulator, which is coupled to thesemiconductor laser, is capable of providing the first modulationcurrent level to the semiconductor laser when the input data signal hasa first logical state, such as a logical one, and providing the secondmodulation current level to the semiconductor laser when the input datasignal has a second logical state, such as a logical zero, with thefirst modulation current level being greater than the second modulationcurrent level.

The photodetector is optically coupled to the semiconductor laser, andis capable of generating a first photodetector current level in responseto the first optical power level and a second photodetector currentlevel in response to the second optical power level. The extinctionratio controller, consisting of a sampler and a modulation currentcontroller, is coupled to the photodetector and to the modulator. Thesampler portion of the extinction ratio controller is capable ofsampling the first photodetector current level to form a firstphotodetector current indicator and sampling the second photodetectorcurrent level to form a second photodetector current indicator. Themodulation current controller portion of the extinction ratio controlleris capable of determining a measured extinction ratio as a ratio of thefirst photodetector current indicator to the second photodetectorcurrent indicator; determining a variance between the measuredextinction ratio and a predetermined extinction ratio and, based on thevariance, forming an extinction ratio error signal; and in response tothe extinction ratio error signal, is further capable of adjusting themodulation current provided by the modulator to the semiconductor laserto generate the optical signal substantially having the predeterminedextinction ratio.

Depending on the bandwidth (or responsiveness) of the photodetector orother component (i.e., how quickly the photodetector settles to anaccurate current level in response to the optical signal), the samplermay also include a timer capable of enabling the sampling of the firstphotodetector current level when the input data signal has apredetermined number of consecutive bits having the first logical stateand enabling the sampling of the second photodetector current level whenthe input data signal has a predetermined number of consecutive bitshaving the second logical state. Alternatively, the timer is capable ofenabling the sampling of the first photodetector current level when theinput data signal has the first logical state for a predetermined periodof time and enabling the sampling of the second photodetector currentlevel when the input data signal has the second logical state for thepredetermined period of time.

In various embodiments, the sampler is further capable of sampling thefirst photodetector current level to form a plurality of firstphotodetector current indicators and sampling the second photodetectorcurrent level to form a plurality of second photodetector currentindicators. For this embodiment, the modulation current controller isfurther capable of determining the measured extinction ratio as a ratioof a first arithmetic mean of the plurality of first photodetectorcurrent indicators to a second arithmetic mean of the plurality ofsecond photodetector current indicators, and determining a variancebetween the measured extinction ratio and the predetermined extinctionratio, and based on the variance, forming the extinction ratio errorsignal.

In response to the extinction ratio error signal in various forms, themodulation current may be controlled in several ways, such as by themodulation current controller providing a modulation adjustment signalto the modulator for the modulator to vary its current levels, or by themodulation current controller directly varying the modulation currentlevels of the modulator. In one embodiment, the modulation currentcontroller is further capable of integrating the extinction ratio errorsignal with a plurality of previous extinction ratio error signals toform an integrated extinction ratio error signal, and is further capableof adjusting the modulation current by providing a selected current pathfor the modulator, of a plurality of varied or variable current paths,with the selected current path corresponding to the integratedextinction ratio error signal. In another embodiment, the modulationcurrent controller is capable of providing, in response to theextinction ratio error signal or an integrated extinction ratio errorsignal, a modulation (current) adjustment signal to the modulator, andin response to the modulation current adjustment signal, the modulatoris capable of adjusting the first modulation current level and thesecond modulation current level for the semiconductor laser to generatethe optical signal having substantially the predetermined extinctionratio.

Numerous other advantages and features of the present invention willbecome readily apparent from the following detailed description of theinvention and the embodiments thereof, from the claims and from theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. (or FIG.) 1 is a block diagram of an exemplary first apparatusembodiment in accordance with the present invention.

FIG. (or FIG.) 2 is a graphical illustration of an exemplary transferfunction for a semiconductor laser.

FIG. (or FIG.) 3 is a block and circuit diagram of an exemplary secondapparatus embodiment in accordance with the present invention.

FIGS. 4A and 4B, individually and collectively referred to as FIG. (orFIG.) 4, are flow diagrams of an exemplary method embodiment inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

While the present invention is susceptible of embodiment in manydifferent forms, there are shown in the drawings and will be describedherein in detail specific embodiments thereof, with the understandingthat the present disclosure is to be considered as an exemplification ofthe principles of the invention and is not intended to limit theinvention to the specific embodiments illustrated.

As mentioned above, the present invention provides an apparatus andmethod for controlling the extinction ratio of a semiconductor laser. Asdiscussed in greater detail below, the present invention provides foradjusting a modulation current of a laser diode to generate an opticaloutput signal having a substantially constant, predetermined extinctionratio. As a consequence, the various embodiments of the presentinvention provide significant compensation for the changingcharacteristics of semiconductor lasers, maintaining the quality of thelaser optical output at predetermined or user selected levels.

FIG. 1 is a block diagram of an exemplary first apparatus embodiment 100in accordance with the present invention. The exemplary apparatus 100may be embodied as an integrated circuit, or as a portion of anintegrated circuit having other, additional components. Typically, theapparatus 100 is embodied with a semiconductor laser and photodetectoras discrete components, and the remainder of the apparatus as anintegrated circuit, with all such discrete or integrated embodimentsconsidered equivalent and within the scope of the present invention.

Referring to FIG. 1, the apparatus 100 includes a semiconductor lasersuch as a laser diode 25, a modulator 10, a photodetector 20, and anextinction ratio controller 15. The modulator 10 and the extinctionratio controller 15 may be considered to comprise a “laser driver” 11which controls the operation of the laser diode 25. The extinction ratiocontroller 15 includes a sampler 60 and a modulation current controller40. The modulator 10 provides a modulation current to the laser diode25, in response to an input data signal (illustrated as “SENT_DATA”),providing a first level of modulation current (i.e., a first modulationcurrent or first modulation current level) when the input data signalhas a first logical state, and a second level of modulation current(i.e., a second modulation current or second modulation current level)when the input data signal has a second logical state. These first andsecond logical states of the input signal, equivalently, may becorresponding voltage levels, and reference to either a logical state orvoltage level shall mean and include reference to the other.

In response to these levels of modulation current (and, as discussedbelow, having an additional forward bias current), the laser diode 25 isoperative to transmit an optical signal corresponding to the input datasignal (with the transmitted or output optical signal representing firstand second logical states referred to as “optical 1's” and “optical0's”, respectively). As mentioned above, when the input data signal is alogical one or high, the optical output signal has a first optical powerlevel, and when the input data signal is a logical zero or low, theoptical output signal has a second optical power level. While the firstoptical power level is generally greater than the second optical powerlevel, alternative logical assignments of data to power levels may beutilized within the scope of the present invention. As discussed ingreater detail below, the amount or level of both first and secondmodulation currents provided by the modulator 10 to the laser diode 25,for the first optical power level and the second optical power level,respectively, is determined by the extinction ratio controller 15.

A semiconductor laser, such as laser diode 25, emits coherent radiationfrom both a forward facet and a rear facet of the active layer. Whilenot separately illustrated, the forward facet of the laser diode 25 istypically optically coupled to a communication or other transmissionmedium, such as an optical fiber, to transmit a data signal (as theoptical output signal). The photodetector 20 is positioned at the rearfacet for illumination by the laser diode 25, so that the photodetector20 generates a corresponding photodetector current used for monitoringof the optical output signal power of the laser diode 25 by theextinction ratio controller 15, as discussed below.

The optical output signal power from the laser diode 25, as indicatedabove, varies between the first optical power level and the secondoptical power level in accordance with the input data signal. Thephotodetector 20, optically coupled to the laser diode 25, converts thevarying optical output signal power into a correspondingly varyingphotodetector current. As a consequence, the photodetector currentvaries with and tracks the laser output signal power, specificallyvarying with the first optical power level and the second optical powerlevel. The extinction ratio controller 15 of the invention, coupled tothe photodetector 20 and the modulator 10, uses this correspondingphotodetector current to determine how the laser diode 25 is performing.More specifically, the sampler 60 samples the photodetector current, andbased on the photodetector current level, the extinction ratiocontroller 15 (through the modulation current controller 40) willdetermine whether the laser diode 25 is providing output signal powerhaving the appropriate, selected or predetermined extinction ratio, andwill adjust the levels of modulation current provided to the laser diode25 accordingly. For example, if the extinction ratio controller 15determines that the currently provided extinction ratio is low, it willincrease either or both the first and second levels of modulationcurrent to drive the laser diode 25 at correspondingly higher powerlevels to achieve the selected or predetermined extinction ratio. Overtime, these adjustments to the modulation current will be in eversmaller increments, and as discussed in greater detail below, will thencause the laser diode 25 to generate and maintain over time an opticaloutput signal having a significantly or substantially constantextinction ratio.

It should be noted that the laser diode current is maintained above thelasing threshold for both the first optical power level (optical 1) andthe second optical power level (optical 0) to avoid an undesirablephenomenon commonly referred to in the art as turn-on time jitter orlaser chirp. The various embodiments of the invention, therefore,include provision of a forward bias current to the laser diode 25, inaddition to the provision of modulation current by the modulator 10.

The modulation current, more specifically, may be considered to be afirst modulation current or first modulation current level, referred toherein as an I_(MOD) (1) current, provided by the modulator 10 to thelaser diode 25 for transmission of an optical 1 (logical one) at thefirst, higher optical power level, and a second modulation current orsecond modulation current level, referred to herein as an I_(MOD) (0)current, provided by the modulator 10 to the laser diode 25 fortransmission of an optical 0 (logical zero) at the second, lower opticalpower level. Therefore, the total current flowing through the laserdiode 25 may be represented as a sum of the DC forward bias current,plus either a first modulation current (I_(MOD) (1) current or currentlevel) or a second modulation current (I_(MOD) (0) current or currentlevel), depending on whether transmission of an optical 1 or an optical0 is required (based on the input data signal). These current levels andcorresponding optical output signal power levels are also illustratedgraphically in FIG. 2, and are discussed below.

FIG. 2 is a graphical illustration of an exemplary transfer function fora semiconductor laser, illustrated as optical output power (L) as afunction of laser diode 25 current (I) (forward bias current plusmodulation current). As illustrated in FIG. 2, the desired orpredetermined extinction ratio is A/C (on line 101), as a ratio of afirst optical power level of “A” to a second optical power level of “C”,using respective (corresponding) current levels of “J” (forward biascurrent plus I_(MOD) (1) current) and “G” (forward bias current plusI_(MOD) (0) current).

It should be noted that there are typically several different ways tocombine the forward bias current and the various modulation currents(I_(MOD) (1) current and I_(MOD) (0) current), namely, through DCcoupling or AC coupling (illustrated with reference to FIG. 3). For DCcoupling (and with reference to the transfer function illustrated asline 101), the forward bias current may be set to a level of “G”,directly providing a current level associated with an optical 0, withI_(MOD) (0) current equal to zero and I_(MOD) (1) current equal to anamount (J-G). Alternatively and equivalently for DC coupling, theforward bias current may be set to a lasing threshold level of “F”, withI_(MOD) (0) current equal to an amount (G-F) and I_(MOD) (1) currentequal to an amount (J-F). Also alternatively and equivalently, for ACcoupling (and with reference to the transfer function illustrated asline 101), the forward bias current may be set to a midpoint level of“M” (with the optical output power level midpoint “E” determined as thearithmetic average of the optical 1 (“A”) level and the optical 0 (“C”)level ((A+C)/2)), with I_(MOD) (0) current equal to a negative value ofmagnitude (M-G) (i.e., −(M-G)), and I_(MOD) (1) current equal to apositive value of magnitude (J-M).

Because laser diode performance varies over time, due to processing,aging, temperature of the laser diode, and so on, optical output powerlevels of the optical 1's and 0's also vary over time. As illustrated(on line 102), optical output power levels have decreased for the samelaser current levels, with the same current levels now providing anextinction ratio of B/D, as a ratio of a first optical power level of“B” to a second optical power level of “D” (also using respectivecurrent levels of “J” and “G”). As will be apparent from FIG. 2, such achange in the laser diode transfer function (laser performance) may beillustrated as a change in the slope of the L vs. I graph of FIG. 2 (andfurther illustrates a change in the modulation currents required toprovide an optical 0 or an optical 1 at a desired optical output powerlevels). In addition, there may also be a change in the thresholdcurrent, from F to F′.

In accordance with the present invention, to accommodate thesevariations in the laser diode characteristics (or transfer functioncharacteristics) which may occur, the change in extinction ratio will bedetermined, as a measured extinction ratio, and adjustments inmodulation current levels by the extinction ratio controller 15 are thenused in order to maintain the extinction ratio at a substantiallyconstant, predetermined level. To maintain the predetermined extinctionratio of A/C, corresponding modulation current levels are changed, usinga first modulation current level (with forward bias current) of “K” forthe first optical power level (forward bias current plus I_(MOD) (1)current) and a second modulation current level (with forward biascurrent) of “H” for the second optical power level (forward bias currentplus I_(MOD) (0) current).

As mentioned above, there are typically several different ways tocombine the forward bias current and the various modulation currents(I_(MOD) (1) current and I_(MOD) (0) current). For DC coupling (and withreference to the transfer function illustrated as line 102), the forwardbias current may be set to a level of “H”, directly providing a currentlevel associated with an optical 0, with I_(MOD) (0) current equal tozero and I_(MOD) (1) current equal to an amount (K-H). Alternatively andequivalently for DC coupling, the forward bias current may be set to alasing threshold level of “F′”, with I_(MOD) (0) current equal to anamount (H-F′) and I_(MOD) (1) current equal to an amount (K-F′). Alsoalternatively and equivalently, for AC coupling (and with reference tothe transfer function illustrated as line 102), the forward bias currentmay be set to a new midpoint level of “N”, with I_(MOD) (0) currentequal to a negative value of magnitude (N-H) (i.e., −(N-H)), and I_(MOD)(1) current equal to a positive value of magnitude (K-N).

As disclosed in the related application, in actual operation, theextinction ratio controller 15 operates in conjunction with an opticalmidpoint controller (not separately illustrated) which provides thefunctionality of the forward bias current generator 65 to determine andprovide the forward bias current for the laser diode 25. For example,such an optical midpoint controller will provide a forward bias currentof F, G or M (for the various alternatives discussed with reference tothe transfer function illustrated as line 101), and will provide aforward bias current of F′, H or N (for the various alternativesdiscussed with reference to the transfer function illustrated as line102). Depending upon the DC or AC embodiment, the extinction ratiocontroller 15 then provides the corresponding modulation currents(I_(MOD) (1) current and I_(MOD) (0) current) discussed above tomaintain a substantially constant, predetermined extinction ratio ofA/C, as illustrated in FIG. 2.

Referring again to FIG. 1, the apparatus 100 utilizes the photodetector20 to detect the optical output signal power from the laser diode 25 andto convert the detected optical output signal power into a corresponding(electrical) photodetector current (i.e., the current through thephotodetector 20 corresponds to and tracks the optical output signal).Using the photodetector current (as a form or type of signal) from thephotodetector 20, the extinction ratio controller 15: (1) measures orotherwise determines the extinction ratio (discussed below in detail)presently being provided by the laser diode 25; (2) compares themeasured extinction ratio with a selected, desired or predeterminedextinction ratio; and (3) if and when the measured extinction ratiovaries from the selected or predetermined extinction ratio, adjusts thefirst and second modulation current levels (via feedback through themodulator 10) to cause the laser diode 25 to generate an optical outputsignal having the predetermined extinction ratio, and over time, havinga substantially constant, predetermined extinction ratio (i.e., thefirst power level and the second power level of the optical outputsignal form a substantially constant, predetermined extinction ratio).

When the extinction ratio controller 15 has determined that the measuredextinction ratio varies from the selected or predetermined extinctionratio, there are innumerable different ways in which feedback to themodulator 10 may be provided to vary the modulation current supplied tothe laser diode 25. In many instances, depending upon the implementationof the modulator 10, the type of feedback selected will varyaccordingly. For example, as discussed below with reference to FIG. 3,when the modulator 10 is implemented as a switching arrangement ordifferential amplifier (of modulator 110), the extinction ratiocontroller 15 is implemented (as extinction ratio controller 115) toprovide a selected current path for the modulator, of a plurality ofvaried or variable current paths, with the selected current pathcorresponding to the desired level of current flow for the modulator,thereby allowing more or less current through the modulator 10 and laserdiode 25 to achieve the predetermined extinction ratio. Other forms ofvarying the modulation of the laser diode 25 current may include, forexample, switching one or more current paths to vary the modulationcurrent, driving associated transistors at various voltage levels toprovide varying current flow (such as varying the voltages at the basesor gates of driving transistors of a modulator), providing variableresistance levels in a current path, and so on. As a consequence, itwill be understood by those of skill in the art that all suchimplementations and their variations are considered equivalents heretoand are within the scope of the present invention.

FIG. 3 is a block and circuit diagram of an exemplary second apparatusembodiment 200 in accordance with the present invention. The exemplarysecond apparatus 200 may be embodied as an integrated circuit, or as aportion of an integrated circuit having other, additional components.Typically, the apparatus 200 is also embodied with a semiconductor laserand photodetector as discrete components, and the remainder of theapparatus as an integrated circuit, with all such discrete or integratedembodiments considered equivalent and within the scope of the presentinvention.

Similarly to the apparatus 100 discussed above, the second apparatus 200also includes a semiconductor laser such as the laser diode 25, aphotodetector 20, an extinction ratio controller 115 and a modulator110. The modulator 110 and the extinction ratio controller 115 also maybe considered to comprise a “laser driver” which controls the operationof the laser diode 25. As illustrated, the second apparatus 200 alsoincludes a forward bias current generator 65, to provide a selected orpredetermined level of forward bias current, as discussed above. Whilethe input data signal (e.g., SENT_DATA) is provided to the modulator 110(on line 18), in this embodiment, the input data signal is also providedas one of the inputs to the extinction ratio controller 115 (namely, tothe timer/CID 50, the inverter 55, and register 35). The extinctionratio controller 115 also has inputs of the photodetector current (online or input 22), along with the input of a value for the predeterminedor selected extinction ratio (as a device setting, on line or input 23),and has an output (on line or output 21) to the modulator 110.

It should be noted that the input of the photodetector current may beviewed or represented equivalently as a voltage level produced by thephotodetector current through a known impedance, such as resistor 96(such as for sampling of the voltage level representing thephotodetector current by analog-to-digital converter (“A/D converter”)30); as a consequence, as used herein, the photodetector current (orphotodetector current level) may be considered to equivalently mean andrefer to the voltage level produced by the photodetector current througha known impedance (e.g., resistor 96). A power supply voltage, Vcc, isprovided to the laser diode 25 on line 56. Also as will be appreciatedby those of ordinary skill in the art, any of a plurality of types ofsemiconductor lasers and photodetectors may be used equivalently herein.

Also as discussed above with reference to FIG. 1, the modulator 110provides a modulation current to the laser diode 25, in response to aninput data signal (on line or input 18), providing a first level ofmodulation current (a first modulation current or I_(MOD) (1)) when theinput data signal has a first logical state, and a second level ofmodulation current (a second modulation current or I_(MOD) (0)) when theinput data signal has a second logical state. In response to theselevels of modulation current (and, having a forward bias current fromgenerator 65), the laser diode 25 is capable of transmitting an opticalsignal (representing first and second logical states (optical 1's andoptical 0's)) corresponding to the input data signal. As mentionedabove, when the input data signal is a logical one or high, the opticaloutput signal has a first optical power level, and when the input datasignal is a logical zero or low, the optical output signal has a secondoptical power level. Again, while the first optical power level isgenerally greater than the second optical power level, alternativelogical assignments of data to optical power levels may be utilizedwithin the scope of the present invention. Also, the amount or level ofboth the first modulation current and the second modulation currentprovided by the modulator 110 to the laser diode 25, for the firstoptical power level and the second optical power level, respectively, isdetermined by the extinction ratio controller 115.

The modulator 110 includes an input data driver (or drive circuit) 95that receives the input data signal, and a differential amplifier 90coupled to an output of the input data drive circuit 95. As illustrated,the bases of the transistors 91 and 92 comprising differential amplifier90 are each coupled to an output of the input data driver 95, receivinginput “N” (as an inverse of the input data (or DATA−)) and input “P” (asthe input data (or DATA+)), which correspond to the first and secondlogic states of the input data signal. For example, as illustrated, theinput data driver 95 converts the input data signal into a form suitablefor use by the differential amplifier 90, such that the differentialamplifier 90 is capable of providing the first modulation current levelto the laser diode 25 when the input data signal has a first logicalstate (logical 1) and the second modulation current to the laser diode25 when the input data signal has a second logical state (logical 0)(e.g., via transistors 91 and 92, as inputs “N” and “P”, respectively).The differential amplifier 90 also may be considered equivalently to bea comparator or a switching arrangement, and numerous other equivalentimplementations of modulator 110 will be apparent to those of skill inthe art, as mentioned above. For purposes of the present invention, themodulator 110 need only provide current to the laser diode 25 for thelaser diode to have appropriate first and second output optical powerlevels, in response to the input data signal. In addition, the modulator110 of the invention should either, depending upon the selectedembodiment: (1) be responsive to any adjustment signal or otherinformation (such as a modulation adjustment signal) from the extinctionratio controller 115 to change the first or second modulation currents,such as, for example, by driving any applicable transistors at adifferent voltage levels to allow different levels of current flow; or(2) as discussed below, be configured to allow the extinction ratiocontroller 115 to directly change these modulation current levels, suchas by providing a selected current path, of a plurality of varied orvariable current paths, with the selected current path corresponding tothe desired level of modulation current to achieve the predeterminedextinction ratio.

The modulator 110 (through differential amplifier 90) may also haveeither an AC or DC coupling to the laser diode 25. As illustrated, whenthe collector of transistor 92 is coupled (via line 57) to the laserdiode 25, DC coupling is provided. Alternatively, in lieu of DC couplingvia line 57, AC coupling may be provided utilizing a capacitor 58coupled between the laser diode 25 and the collector of transistor 92and utilizing an inductor 59 coupled between the power supply voltage(Vcc on line 56) and the collector of the transistor 92, as illustratedwith the dotted lines in FIG. 3.

As in FIG. 1, the sampler 160 of the extinction ratio controller 115 ofFIG. 3 has an input of the photodetector current (as a voltage level)from the photodetector 20, and in addition, an input of the input datasignal. The extinction ratio controller 115 measures or determines apresently (i.e., currently) occurring extinction ratio (discussed belowin detail), and if the measured extinction ratio varies from thepredetermined extinction ratio, adjusts the modulation current to causethe laser diode to generate an optical output signal having thepredetermined extinction ratio. The measured (or determined) extinctionratio, as a numerical value, is based on (1) a currently measured,optical power level, and (2) a previously measured and stored opticalpower level that represents a logical opposite of the currently measuredoptical power level. For example, when the currently measured opticaloutput signal is at or about the first optical power level andrepresents a logical or optical one, then the previously measured andstored optical power level which is used to form the measured extinctionratio is for an optical output signal that is at or about the secondoptical power level and which represents a logical or optical zero.Similarly, when the currently measured optical output signal is at orabout the second optical power level and represents a logical or opticalzero, then the previously measured and stored optical power level whichis used to form the measured extinction ratio is for an optical outputsignal that is at or about the first optical power level and whichrepresents a logical or optical one.

The extinction ratio controller 115 includes a sampler 160 (equivalentlya voltage or a current sampler) and a modulation current controller 140.The sampler 160 includes an analog-to-digital converter (“A/Dconverter”) 30 coupled to the photodetector 20, a first (high) holdingregister 35 coupled to an output of the A/D converter 30, a second (low)holding register 45 coupled to the output of the A/D converter 30, atimer/CID 50, and an inverter 55. The A/D converter 30 receives as inputthe photodetector current (generally as a voltage, as discussed above).When indicated (or enabled) by the timer/CID 50 (as discussed below),the A/D converter 30 samples the photodetector current (generally, as acorresponding voltage level) and converts the sampled photodetectorcurrent (or voltage) into a digital form or digital value, referred toas a photodetector current indicator.

When enabled or indicated by the timer/CID 50, the A/D converter 30provides a photodetector current indicator, as a digital value,corresponding to the photodetector current sample at that instant intime. As mentioned above, equivalently, the photodetector current may bemonitored and sampled as a corresponding voltage level. Thephotodetector current at that instant in time corresponds to the opticaloutput power of either a first logical state (logical 1) or a secondlogical state (logical 0). The timer/CID 50 indicates (as a timingsignal, indicator or enable (illustrated as enable “S”)) to the A/Dconverter 30 that the photodetector current should be sampled (andconverted to a photodetector current indicator) when the input datasignal indicates that a transition between first and second logicalstates has not occurred for a predetermined time period or for apredetermined number of bits (discussed below). When there has not beensuch a transition for a predetermined period of time or a predeterminednumber of bits (e.g., a certain number of consecutive bits have beenidentical, either all first logical states (1's) or all second logicalstates (0's)), the A/D converter 30 then samples the photodetectorcurrent and converts the photodetector current sample into a digitalform, referred to as a photodetector current indicator, suitable for useby the modulation current controller 140. Alternatively, the A/Dconverter 30 may continually sample the photodetector current, andconvert the sampled photodetector current to a digital form, as aphotodetector current indicator, only when similarly enabled by thetimer/CID 50.

The timing indication or enable signal from the timer/CID 50 to samplethe photodetector current and/or convert the sample to a photodetectorcurrent indicator is based on the “settling” time (or time to reach asteady-state) of the photodetector and sampler in the apparatus 200,which may be measured as a duration of time or as a sequential number oftransmitted bits. Typically, the slowest component will be thephotodetector 20, which requires a particular amount of time to receivelight at either the first optical power level or the second opticalpower level to then produce a consistent, corresponding photodetectorcurrent which is representative of each of these power levels. As aconsequence, the timer/CID 50 is set for the photodetector 20 and A/Dconverter 32 (of sampler 160) to settle, and is independent of the speedof the other components of the apparatus 200 illustrated in FIG. 3. Thetimer/CID 50 may be configured to determine either that a sufficientnumber of successive bits of the same logical state (consecutive logical1's or logical 0's, as consecutive identical digits or “CIDs”) have beenreceived by the photodetector 20 (as corresponding to the input datasignal), or that a sufficient period of time has elapsed for the receiptby the photodetector 20 of consecutive logical 1's or logical 0's. Itshould be noted that this timing or counting process is applicable toany slowest component, which may or may not be the photodetector 20.This use of the timer/CID 50, however, does permit the use of aphotodetector 20 which is comparatively slower and has a comparativelymore limited bandwidth (and, generally, may also be implemented at lowercost).

As a consequence, when either a predetermined period of time has elapsedor transmission of a predetermined number of bits having the samelogical state (CID) has occurred, the timer/CID 50 will provide acorresponding indication or signal, referred to as a timing signal ortiming enable. This timing signal is provided to a plurality of thecomponents of the extinction ratio controller 115, as illustrated,namely, the A/D converter 30, the registers 35, 45, and the extinctionratio integrator 80. When indicated by the timer/CID 50, the digitalvalue representing a sample of a settled photodetector output currentthen provides a basis for accurate measurement of the actual extinctionratio being provided by the laser diode 25. As a consequence, the timingsignal is provided to enable the A/D converter 30, the registers 35, 45,and the extinction ratio integrator 80, thereby providing that thesecomponents are enabled only when the current sample is consideredaccurate and valid. In the absence of the timing signal or timingenable, these components do not provide additional data for use indetermining a measured extinction ratio and controlling the modulationcurrent, such that the modulation current then remains at its previouslevels (i.e., the modulation current remains at its most recent firstand second modulation current levels).

As indicated above, the photodetector current indicator corresponds toeither a first optical power level representative of a first logicalstate (logical 1) or a second optical power level representative of asecond logical state (logical 0). Given that at least one photodetectorcurrent indicator for the first optical power level and at least onephotodetector current indicator for the second optical power level areneeded to determine the extinction ratio, as these values are determinedover time based on the data transmitted, the values are stored inseparate, corresponding registers, with a first holding register 35storing one or more photodetector current indicators of the firstoptical power level and with a second holding register 45 storing one ormore photodetector current indicators of the second optical power level.As the photodetector current indicator from the A/D converter 30 may befor either the first optical power level or the second optical powerlevel, the input data signal is utilized (with inverter 55) tocorrespondingly enable either the first holding register 35 or thesecond holding register 45 (via inverter 55) to store the photodetectorcurrent indicator (illustrated as enable “E”). For ease of reference,the photodetector current indicator corresponding to the first opticalpower level is referred to as a first photodetector current indicator,and the photodetector current indicator corresponding to the secondoptical power level is referred to as a second photodetector currentindicator. For example, when enabled by the timing signal from thetimer/CID 50, the digital value for the photodetector current sample isstored in the first holding register 35 when enabled by the input datasignal (as indicating a logical 1), as a first photodetector currentindicator, or is stored in the second holding register 45 when enabledby the inverted input data signal (as indicating a logical 0 which hasbeen inverted by inverter 55), as a second photodetector currentindicator. Depending upon the selected embodiment, the first holdingregister 35 may store a plurality of samples of the photodetectorcurrent for the first logical state, as a plurality of firstphotodetector current indicators, and the second holding register 45 maystore a plurality of samples of the photodetector current for the secondlogical state, as a plurality of second photodetector currentindicators.

Continuing to refer to FIG. 3, the modulation current controller 140comprises an extinction ratio calculator 70, an extinction ratio errorgenerator 75, an extinction ratio integrator 80, and a digital-to-analogconverter (“DAC”) 85.

The extinction ratio calculator 70 is coupled to the first and secondholding registers 35, 45, and determines the measured extinction ratioof the laser diode 25. As discussed in greater detail below, theextinction ratio calculator 70 may determine the measured extinctionratio as: (1) a ratio of a singular first photodetector currentindicator to a singular second photodetector current indicator (i.e.,forming a ratio using a single sample of each of the photodetectorcurrent indicators which correspond to the first optical power level andthe second optical power level); or (2) as a ratio of a first averagevalue or arithmetic mean of a plurality of first photodetector currentindicators to a second average value or arithmetic mean of a pluralityof second photodetector current indicators (i.e., forming a ratio usinga plurality of samples of each of the photodetector current indicatorscorresponding to the first optical power level and the second opticalpower level).

The extinction ratio error generator 75 has a first input coupled to theextinction ratio calculator 70, and has a second input (on line 23) toreceive a predetermined or preset extinction ratio value which has beenselected or which is desired for the optical output power levels of thelaser diode 25. The extinction ratio error generator 75 will determine adifference (or other variance), if any, between the predeterminedextinction ratio and the measured extinction ratio, and provide acorresponding extinction ratio error signal. For example, the extinctionratio error generator 75 may be implemented as an adder (subtracter) orsummer, subtracting the measured extinction ratio from (or adding aninverted measured extinction ratio to) the predetermined extinctionratio. The extinction ratio error signal has both a magnitude, as theabsolute value of any difference between the predetermined extinctionratio and the measured extinction ratio, and has a sign, such that itwill be positive when the measured extinction ratio is less than thepredetermined extinction ratio, and will be negative when the measuredextinction ratio is greater than the predetermined extinction ratio.

Subject to the various caveats discussed below, the extinction ratioerror (signal) will then provide the basis for determining or varyingthe first and second modulation current levels of the modulator 110 tomaintain a substantially constant, predetermined extinction ratio (i.e.,a substantially constant extinction ratio at a predetermined level). Howthe extinction ratio error is utilized will depend upon the selectedembodiment of the modulator 110, as mentioned above. For example, in anembodiment in which the modulator were to provide the first and secondmodulation currents directly to the laser diode 25, the extinction ratioerror signal (or as discussed below, its integrated form, as anintegrated extinction ratio error signal) may be provided directly tothe modulator, as a modulation current adjustment signal, for use by themodulator in directly varying the first and second modulation currentsof the laser diode 25 to provide a substantially constant, predeterminedextinction ratio.

For the illustrated embodiment, however, the extinction ratio controller115 determines the overall or total amount of current through themodulator 110 and, in particular, through the differential amplifier (orswitching arrangement) 90. Within the modulation current controller 140,the DAC 85 is coupled to the modulator 110 and provides this currentcontrol function; because the modulator 110 must always have at leastsome current to modulate the laser diode 25 in response to the inputdata signal, in this embodiment the DAC 85 always provides at least somecurrent to the modulator 110, with the amount of current dependent upon,among other things, the predetermined extinction ratio and theextinction ratio error signal. To force the steady state extinctionratio error towards zero and filter the extinction ratio error signal tominimize control loop effects on the transmitted signal, the extinctionratio error signal is not provided directly to the modulator 110 to, forexample, directly vary first and second modulation currents. Rather, theextinction ratio error signal is integrated (or summed) with previouserror values by extinction ratio integrator 80. This integration of theextinction ratio error signal with previous error values by extinctionratio integrator 80, to form an integrated extinction ratio errorsignal, is utilized by the DAC 85 to determine an overall or total levelof modulation current for the modulator 110, and provides that acorresponding positive, but variable, modulation current level isprovided by the DAC 85 to the modulator 110.

As mentioned above, the integration by the extinction ratio integrator80 is enabled by the timing signal (timing enable), such that a validextinction ratio error signal is utilized before a change may be made tothe integrated extinction ratio error signal, with corresponding effectson the modulation current levels. The DAC 85, in turn, as a variablecurrent source/sink, then provides that indicated level of modulationcurrent, corresponding to the integrated extinction ratio error signal,to the modulator 110. More particularly, using the integrated extinctionratio error signal, the DAC 85 converts the integrated extinction ratioerror signal to a corresponding analog value indicative of theappropriate level of modulation current, and provides a corresponding,selected current path (or current sink) from the modulator 110 to aground potential (of a plurality of varied or variable current paths),and thereby allows a corresponding level of modulation current to flowthrough the modulator 110 in order for the first optical power level andthe second optical power level of the laser diode 25 to have thepredetermined extinction ratio. For example, the DAC 85 may provide aplurality of switchable current paths having differingimpedance/resistance levels, and using control provided by theintegrated extinction ratio error signal, will therefore providedifferent levels of current capacity when switched in or out of the pathbetween the modulator 110 and ground, e.g., with lower resistance pathsallowing increased modulation current levels, and so on. It will beapparent to those of skill in the art that myriad other equivalent DAC85 implementations are also available.

It will be apparent to those of skill in the art that when the modulator110 may be implemented using other equivalent arrangements, the DAC 85(as a variable current source or sink) will be varied accordingly and,in some cases, the extinction ratio error may or may not need to beintegrated or summed prior to being used to indicate or providemodulation current levels. In such an equivalent embodiment, when anentity separate from a modulator (such as the DAC 85) is not providingall of the current to the modulator, the extinction ratio error signalmay be applied directly to a modulator (for the modulator to vary itsown current levels, as mentioned above), as or as part of a modulationadjustment signal. Numerous other equivalent variations also will beapparent to those of skill in the art, and are within the scope of thepresent invention.

As a consequence, using the first and second photodetector currentindicators (as digital values of photodetector current samples), orusing corresponding averages of a plurality of each of the first andsecond photodetector current indicators, the modulation currentcontroller 140 determines or measures the extinction ratio of the laserdiode 25 (through extinction ratio calculator 70), and compares (ordetermines the variance between) the measured extinction ratio and thepredetermined extinction ratio (through extinction ratio error generator75). In the event the measured extinction ratio varies from thepredetermined extinction ratio, an extinction ratio error signal isgenerated, and is used to vary the modulation current accordingly. Inthe illustrated embodiment, the extinction ratio error signal is used tovary the amount of modulation current provided by the DAC 85 to themodulator 110, by providing an indicator of the total or overallmodulation current level to be provided, as the integrated extinctionratio error signal from the extinction ratio integrator 80. As aconsequence, due to this feedback arrangement, any variance between themeasured extinction ratio and the predetermined extinction ratio isforced toward zero and becomes negligible, resulting in the laser diode25 having a substantially constant extinction ratio at the predeterminedor selected level, namely, a substantially constant and predeterminedextinction ratio.

It should be noted that the various apparatuses 100 and 200 may beimplemented using a wide range of equivalents. For purposes of exampleand not limitation, the extinction ratio controller may be implementedutilizing: an A/D converter or other sampler; any type of memory (suchas random access memory (RAM), flash, DRAM, SRAM, SDRAM, MRAM, FeRAM,ROM, EPROM or E²PROM); a microprocessor (or digital signal processor,controller or other processor) programmed or configured to determine themeasured extinction ratio, error, integrated error, and so on; and avariable current source or varied and switchable current paths; whilethe modulator, laser diode and photodetector may be implemented usingknown architectures and components, in addition to those illustrated.Numerous other equivalent implementations also will be apparent to thoseof skill in the art.

FIGS. 4A and 4B, individually and collectively referred to as FIG. 4,are flow diagrams of an exemplary method embodiment 300 in accordancewith the present invention, and provides a useful summary. In theexemplary embodiment, the method is performed on a continual basisduring operation of the apparatus 100 or apparatus 200, beginning withsystem or apparatus start up (step 305), adjusting the modulationcurrent flowing through the laser diode 25, as needed, in order togenerate an optical output signal having a substantially constant,predetermined extinction ratio. In addition, the illustrated methodembodiment 300 utilizes the continual sampling of the photodetectorcurrent by the A/D converter 30, as mentioned above (rather than theother alternative of sampling only when specifically enabled by thetiming signal).

Referring to FIG. 4, the photodetector current is sampled on acontinuous basis, step 310, by the A/D converter 30. Concurrently, instep 315, the method determines whether an input data transition hasoccurred, in which successive or consecutive bits of the input datasignal transition from one logical state to another logical state (e.g.,a bit transition from a logical 0 to a logical 1 or vice-versa). Whenthe input data transitions from one logical state to another in step315, the timer/CID 50 is reset or initialized, to begin timing orcounting from zero or another equivalent reference or initializationpoint (step 320). The method then determines whether a predetermined orminimum time period has elapsed (or a predetermined consecutiveidentical bit count has been reached) without another (or next) datatransition, step 325, allowing for steady-state (or settled) levels tobe reached, such as for the photodetector current. If the predeterminedor minimum time period or bit count has not been reached before anotherdata transition has occurred in step 325, the method returns to steps310 and 315, to reset the timer or counter while continually samplingthe photodetector current.

When the predetermined or minimum time period or bit count has beenreached without another data transition in step 325, the sample of thephotodetector current (from step 310) is converted to a digital form, asa photodetector current indicator, step 330. For example, the timer/CID50 may “time out” and provide a timing or enable signal, indicating thatthe present photodetector current sample should be converted to digitalform and used in subsequent extinction ratio and error determinations.As mentioned above, the predetermined time period or bit count isselected to allow the components of the apparatus 100 or 200 to settle,if needed. Although the photodetector 20 (with its limited bandwidth) istypically the slowest component to settle, there may be instances wherethe speed of the photodetector 20 is not a factor and therefore use ofthe timer/CID 50 is optional (i.e., the operating speed of thephotodetector 20 is substantially close to the bit rate of the opticaloutput signal).

As mentioned above, when the photodetector current is not sampledcontinually, then following the timing enablement (of step 325), step330 will include both sampling the photodetector current and convertingthe sample of the photodetector current to a digital value or form, as aphotodetector current indicator, for use in the various extinction ratiodeterminations. Next, in step 335, using the input data signal, if thedigital sample of the photodetector current is representative of thefirst logical state (logical 1), such that the sample corresponds to thefirst optical output power level, the corresponding digital value isstored in the first holding register 35, step 340, as a firstphotodetector current indicator. If the digital sample of thephotodetector current is not representative of the first logical state(logical 1) in step 335, and is therefore representative of the secondlogical state (logical 0), the corresponding digital value is stored inthe second holding register 45, step 345, as a second photodetectorcurrent indicator. As a consequence, either a comparatively high or lowdigital value is stored in its corresponding holding register, inresponse to a timing (or enable) signal from the timer/CID 50, and inresponse to the logical state of the input data signal. It should benoted that over time, in previous iterations (with the exception of thefirst several iterations following system start up), another digitalsample of the photodetector current, which is representative of alogical opposite, will have been stored previously in the other holdingregister (i.e., a second photodetector current indicator in the secondholding register 45 or a first photodetector current indicator in thefirst holding register 35).

Using the first photodetector current indicator from the first holdingregister 35 (which corresponds to a first or high optical power leveland represents a first logical state) and the second photodetectorcurrent indicator from the second holding register 45 (which correspondsto a second or low optical power level and represents a second logicalstate), the method determines (step 350) a measured extinction ratio(using extinction ratio calculator 70) by forming a ratio, namely,dividing the first photodetector current indicator by the secondphotodetector current indicator. In a first embodiment, each of thefirst and second holding registers 35, 45 may store only a singledigital sample of the photodetector current per register, so that themeasured extinction ratio is then calculated using one firstphotodetector current indicator and one second photodetector currentindicator.

In another embodiment, the first and second holding registers 35, 45 mayeach store a plurality of samples of the photodetector current, namely,a plurality of first photodetector current indicators and a plurality ofsecond photodetector current indicators. In that event, in step 350, theextinction ratio calculator 70 may determine the measured extinctionratio as a ratio of a first average value (or arithmetic mean) of theplurality of first photodetector current indicators to a second averagevalue (or arithmetic mean) of the plurality of second photodetectorcurrent indicators. Use of these averaged digital values may improve thenoise immunity of the operation of the apparatus 100 or 200.

Regardless of whether the measured extinction ratio value (determined bythe extinction ratio calculator 70) is based upon single samples of eachof the first and second photodetector current indicators, or mean valuesof a plurality of samples, the measured extinction ratio (of step 350)is then compared, by the extinction ratio error generator 75, to thepredetermined or otherwise selected or desired extinction ratio value,to generate an extinction ratio error (step 355). The extinction ratioerror represents the variance or difference (if any) between themeasured extinction ratio determined from the optical output power ofthe laser diode 25 and the predetermined extinction ratio value. Itshould be noted that the extinction ratio error, as discussed above, hasa sign and a magnitude (substantially equal to the variance ordifference between the measured extinction ratio and the predeterminedextinction ratio), and that in the event the difference or variance iszero, the extinction ratio error is also zero. It should also be notedthat over time, the extinction ratio error will be forced to zero, alsoas discussed above, such that the laser diode output power levelsprovide a substantially constant, predetermined extinction ratio.Selection of the predetermined extinction ratio may be based on a numberof factors, including receiver sensitivity, optical transmission loss,noise environment, etc. If the measured extinction ratio is less thanthe predetermined extinction ratio, the extinction ratio error isrepresented as a positive value, and if more than the predeterminedextinction ratio, the extinction ratio error value is represented as anegative value.

Depending upon the modulator 110 (or modulator 10) configuration, theextinction ratio error, if any, may be used directly to correspondinglyvary the modulation current of the laser diode 25, step 365, to providea substantially constant, predetermined extinction ratio. As mentionedabove, for the illustrated apparatus 200 embodiment, an additional stepis utilized, in which the extinction ratio error, as a digital value, issummed or integrated with a plurality of previous extinction ratioerrors (as a sum of a plurality of digital values from previousiterations), to form an integrated extinction ratio error, step 360.More specifically, the present extinction ratio error is effectivelyadded to (or subtracted from) the previous integrated extinction ratioerror, as a running sum. For example, if a first extinction ratio erroris equivalent to +0.3 and a second extinction ratio error is equivalentto −0.08, then the integrated extinction ratio error becomes +0.22; if athird extinction ratio error is equivalent to +0.01, the integratedextinction ratio error will be +0.23; and if a fourth extinction ratioerror is equivalent to 0.00, the integrated extinction ratio error willcontinue to be +0.23. For the illustrated embodiment, the integratedextinction ratio error is then used to correspondingly vary or provide(through DAC 85) the modulation current of the laser diode 25, step 365,to provide a substantially constant, predetermined extinction ratio.Depending upon the selected embodiment, the integrated extinction ratioerror signal may be converted (e.g., scaled and converted to an analogform), as an indicator of the desired modulation current level, for useby the DAC 85 (or use directly by a modulator) to provide acorresponding level of modulation current. Again, as the extinctionratio error tends toward zero, it will be apparent that over time, anyadjustment in step 365 will also become negligible or zero, with themodulation current maintained at previous levels. For the apparatus 200,of a plurality of varied or variable current paths, the DAC 85 providesa corresponding, selected current path to ground for the modulator,providing a modulation current level corresponding to the integratedextinction ratio error. The method then returns to steps 310 and 315, tocontinually monitor and determine the measured extinction ratio of thelaser diode 25, and provide any corresponding adjustments to themodulation current.

In summary, the method of controlling an extinction ratio of asemiconductor laser, in accordance with the present invention, includes:(a) modulating the semiconductor laser at a first modulation level whenthe input data signal has a first logical state and modulating thesemiconductor laser at a second modulation level when the input datasignal has a second logical state; (b) transmitting an optical signalhaving a first optical power level in response to the first modulationlevel and having a second optical power level in response to the secondmodulation level, the first optical power level being greater than thesecond optical power level; (c) detecting the first optical power leveland the second optical power level; (d) determining a measuredextinction ratio as a ratio of the detected first optical power level tothe detected second optical power level; (e) determining an extinctionratio error as a variance between the measured extinction ratio and apredetermined extinction ratio; and (f) using the extinction ratioerror, adjusting the modulation of the semiconductor laser to generatethe optical signal substantially having the predetermined extinctionratio.

The adjustment step (step (f)) may also include integrating theextinction ratio error with a plurality of previous extinction ratioerrors to form an integrated extinction ratio error; and eitheradjusting the modulation of the semiconductor laser in response to theintegrated extinction ratio error (or integrated extinction ratio errorsignal), or adjusting the modulation of the semiconductor laser byproviding a selected current path for the modulator, with the selectedcurrent path corresponding to the integrated extinction ratio error(signal).

The optical signal detection step (step (c)) may be performed bydetecting the first optical power level by sampling a firstphotodetector current generated by the first optical power level to forma first photodetector current indicator and detecting the second opticalpower level by sampling a second photodetector current generated by thesecond optical power level to form a second photodetector currentindicator. This sampling of the first photodetector current and thesecond photodetector current may be performed by sampling correspondingvoltage levels.

As discussed above, the measured extinction ratio may be determined as aratio the first photodetector current indicator to the secondphotodetector current indicator, or as a ratio of a first arithmeticmean of a plurality of samples of the first photodetector current to asecond arithmetic mean of a plurality of samples of the secondphotodetector current. In addition, the photodetector current levels aresampled when the input data signal has a predetermined number ofconsecutive identical bits (having either the first or the secondlogical state), or when the input data signal has one (first or second)logical state for a predetermined period of time.

As mentioned above, the feedback provided by the present invention willgenerally force the extinction ratio error to zero, providing amodulation current such that the laser diode 25 provides a substantiallyconstant, predetermined extinction ratio, with minimal furtheradjustment of the modulation current unless and until laser operatingconditions or characteristics change. As will be appreciated by those ofordinary skill in the art, the DAC 85 or an equivalent device may beconfigured in any number of ways to adjust or provide the modulationcurrent through the modulator 110 or modulator 10 and the laser diode25.

For example, if the measured extinction ratio is less than thepredetermined or selected extinction ratio, providing a corresponding,positive extinction ratio error, the integrated extinction ratio errorwill correspondingly increase, the DAC 85 will provide for acorrespondingly increased current path, which in turn, will allow morecurrent through the modulator 10 and laser diode 25. This will result ina larger measured extinction ratio value, and a smaller extinction ratioerror in subsequent iterations. If the measured extinction ratio valueis greater than the predetermined or selected extinction ratio,providing a negative extinction ratio error, the integrated extinctionratio error will correspondingly decrease, the DAC 85 will provide for acorrespondingly decreased current path, which in turn, will allow lesscurrent through the modulator 110 and laser diode 25. This will resultin a smaller measured extinction ratio value, and a smaller extinctionratio error in subsequent iterations. In this way, operation of theextinction ratio controller 115 (via closed loop feedback) adjusts thecurrent through the laser diode 25 until the extinction ratio error issubstantially zero and the laser diode 25 provides a substantiallyconstant, predetermined extinction ratio.

Another advantage of the present invention concerns use of the timer/CID50 to control the timing of calculations and the corresponding bandwidthrequired to implement the invention. Although the methodology runs on acontinuous basis, the measured extinction ratio is not calculatedcontinuously, but only when a predetermined number of consecutiveidentical digits have been transmitted or the same digit has beentransmitted for a predetermined period of time (based on the input datasignal). As a consequence, the present invention may accommodate a lowerbandwidth (compared to the frequency of data transitions or data rate)of components such as the photodetector, while maintaining accuratefeedback and significant control of the semiconductor laser extinctionratio.

Yet additional advantages of the present invention may be furtherapparent to those of skill in the art. The various embodiments of thepresent invention may be advantageously combined with other aspects oflaser control. For example, as illustrated in the related application,the various embodiments of the present invention may be combined withembodiments for laser midpoint control.

From the foregoing, it will be observed that numerous variations andmodifications may be effected without departing from the spirit andscope of the novel concept of the invention. It is to be understood thatno limitation with respect to the specific methods and apparatusillustrated herein is intended or should be inferred. It is, of course,intended to cover by the appended claims all such modifications as fallwithin the scope of the claims.

1. An integrated circuit, the integrated circuit couplable to asemiconductor laser and to a photodetector, the photodetector opticallycouplable to the semiconductor laser, the semiconductor laser capable oftransmitting an optical signal in response to a modulation current, andthe photodetector capable of converting the optical signal into aphotodetector current, the integrated circuit comprising: a modulatorcouplable to the semiconductor laser, the modulator capable of providingthe modulation current to the semiconductor laser, the modulationcurrent corresponding to an input data signal; and an extinction ratiocontroller couplable to the photodetector and coupled to the modulator,the extinction ratio controller, in response to the photodetectorcurrent, capable of adjusting the modulation current provided by themodulator to the semiconductor laser to generate the optical signalhaving substantially a predetermined extinction ratio.
 2. The integratedcircuit of claim 1, wherein the modulator is capable of providing afirst modulation current level to the semiconductor laser when the inputdata signal has a first logical state and providing a second modulationcurrent level to the semiconductor laser when the input data signal hasa second logical state, the first modulation current level being greaterthan the second modulation current level; wherein the semiconductorlaser is capable of providing the optical signal having a first opticalpower level in response to the first modulation current level and havinga second optical power level in response to the second modulationcurrent level, the first optical power level being greater than thesecond optical power level; and wherein the photodetector is furthercapable of generating a first photodetector current level in response tothe first optical power level and a second photodetector current levelin response to the second optical power level.
 3. The integrated circuitof claim 2, wherein the extinction ratio controller is further capableof: sampling the first photodetector current level to form a pluralityof first photodetector current indicators; sampling the secondphotodetector current level to form a plurality of second photodetectorcurrent indicators; determining a measured extinction ratio as a ratioof a first arithmetic mean of the plurality of first photodetectorcurrent indicators to a second arithmetic mean of the plurality ofsecond photodetector current indicators; determining a variance betweenthe measured extinction ratio and the predetermined extinction ratio,and based on the variance, forming an extinction ratio error signal. 4.The integrated circuit of claim 3, wherein the extinction ratiocontroller is further capable of integrating the extinction ratio errorsignal with a plurality of previous extinction ratio error signals toform an integrated extinction ratio error signal; and wherein theextinction ratio controller is further capable of adjusting themodulation current by providing a selected current path for themodulator, the selected current path corresponding to the integratedextinction ratio error signal.
 5. The integrated circuit of claim 2,wherein the extinction ratio controller is capable of sampling the firstphotodetector current level to form a first photodetector currentindicator, sampling the second photodetector current level to form asecond photodetector current indicator, determining a measuredextinction ratio as a ratio of the first photodetector current indicatorto the second photodetector current indicator, determining a variancebetween the measured extinction ratio and the predetermined extinctionratio and, based on the variance, forming an extinction ratio errorsignal.
 6. The integrated circuit of claim 5, wherein the extinctionratio controller is capable of sampling the first photodetector currentlevel and the second photodetector current level by samplingcorresponding voltage levels.
 7. The integrated circuit of claim 5,wherein the extinction ratio controller is enabled to sample the firstphotodetector current level when the input data signal has apredetermined number of consecutive bits having the first logical stateand is enabled to sample the second photodetector current level when theinput data signal has a predetermined number of consecutive bits havingthe second logical state.
 8. The integrated circuit of claim 5, whereinthe extinction ratio controller is enabled to sample the firstphotodetector current level when the input data signal has the firstlogical state for a predetermined period of time and is enabled tosample the second photodetector current level when the input data signalhas the second logical state for the predetermined period of time. 9.The integrated circuit of claim 5, wherein the extinction ratiocontroller is further capable of integrating the extinction ratio errorsignal with a plurality of previous extinction ratio error signals toform an integrated extinction ratio error signal; and wherein theextinction ratio controller is further capable of adjusting themodulation current in response to the integrated extinction ratio errorsignal.
 10. The integrated circuit of claim 5, wherein the extinctionratio controller is further capable of integrating the extinction ratioerror signal with a plurality of previous extinction ratio error signalsto form an integrated extinction ratio error signal; and wherein theextinction ratio controller is further capable of adjusting themodulation current by providing a selected current path for themodulator, the selected current path corresponding to the integratedextinction ratio error signal.
 11. The integrated circuit of claim 5,wherein the extinction ratio controller is further capable of providing,in response to the extinction ratio error signal, a modulation currentadjustment signal to the modulator, and wherein, in response to themodulation current adjustment signal, the modulator is further capableof adjusting the first modulation current level and the secondmodulation current level for the semiconductor laser to generate theoptical signal having a substantially constant, predetermined extinctionratio.
 12. The integrated circuit of claim 2, wherein the extinctionratio controller further comprises: a sampler coupled to thephotodetector, the sampler capable of sampling the first photodetectorcurrent level to form a plurality of first photodetector currentindicators and sampling the second photodetector current level to form aplurality of second photodetector current indicators; and a modulationcurrent controller coupled to the sampler and to the modulator, themodulation current controller capable of determining a measuredextinction ratio as a ratio of a first arithmetic mean of the pluralityof first photodetector current indicators to a second arithmetic mean ofthe plurality of second photodetector current indicators, and comparingthe measured extinction ratio to the predetermined extinction ratio toform an extinction ratio error signal.
 13. The integrated circuit ofclaim 2, wherein the extinction ratio controller further comprises: asampler coupled to the photodetector, the sampler capable of samplingthe first photodetector current level to form a first photodetectorcurrent indicator and sampling the second photodetector current level toform a second photodetector current indicator; and a modulation currentcontroller coupled to the sampler and to the modulator, the modulationcurrent controller capable of determining a measured extinction ratio asa ratio of the first photodetector current indicator to the secondphotodetector current indicator, and further capable of comparing themeasured extinction ratio to the predetermined extinction ratio to forman extinction ratio error signal.
 14. The integrated circuit of claim13, wherein the sampler further comprises: an analog-to-digitalconverter coupled to the photodetector, the analog-to-digital convertercapable of sampling the first photodetector current level to form afirst photodetector current indicator and sampling the secondphotodetector current level to form a second photodetector currentindicator; and a timer coupled to the analog-to-digital converter, thetimer capable of enabling the analog-to-digital converter to sample thefirst photodetector current level when the input data signal has apredetermined number of consecutive bits having the first logical stateand enabling the analog-to-digital converter to sample the secondphotodetector current level when the input data signal has apredetermined number of consecutive bits having the second logicalstate.
 15. The integrated circuit of claim 13, wherein the samplerfurther comprises: an analog-to-digital converter coupled to thephotodetector, the analog-to-digital converter capable of sampling thefirst photodetector current level to form a first photodetector currentindicator and sampling the second photodetector current level to form asecond photodetector current indicator; and a timer coupled to theanalog-to-digital converter, the timer capable of enabling theanalog-to-digital converter to sample the first photodetector currentlevel when the input data signal has the first logical state for apredetermined period of time and enabling the analog-to-digitalconverter to sample the second photodetector current level when theinput data signal has the second logical state for the predeterminedperiod of time.
 16. The integrated circuit of claim 13, wherein thesampler further comprises: a first register coupled to theanalog-to-digital converter, the first register capable of storing thefirst photodetector current indicator when the input data signal has afirst logical state; and a second register coupled to theanalog-to-digital converter, the second register capable of storing thesecond photodetector current indicator when the input data signal has asecond logical state.
 17. The integrated circuit of claim 13, whereinthe modulation current controller further comprises: an extinction ratiocalculator coupled to the sampler, the extinction ratio calculatorcapable of determining a measured extinction ratio as a ratio of thefirst photodetector current indicator to the second photodetectorcurrent indicator; and an extinction ratio error generator coupled tothe extinction ratio calculator, the extinction ratio error generatorcapable of determining a variance between the measured extinction ratioand the predetermined extinction ratio and, corresponding to thevariance, forming the extinction ratio error signal.
 18. The integratedcircuit of claim 17, wherein the modulation current controller furthercomprises: an extinction ratio integrator coupled to the extinctionratio error generator, the extinction ratio integrator capable ofsumming the extinction ratio error signal with a plurality of previousextinction ratio error signals to form an integrated extinction ratioerror signal; and a digital-to-analog converter coupled to theextinction ratio integrator, the digital-to-analog converter capable ofadjusting the modulator current by providing a selected current path forthe modulator, the selected current path corresponding to the integratedextinction ratio error signal.
 19. A method of controlling an extinctionratio of a semiconductor laser, the method comprising: (a) modulatingthe semiconductor laser at a first modulation level when the input datasignal has a first logical state and modulating the semiconductor laserat a second modulation level when the input data signal has a secondlogical state; (b) transmitting an optical signal having a first opticalpower level in response to the first modulation level and having asecond optical power level in response to the second modulation level,the first optical power level being greater than the second opticalpower level; (c) detecting the first optical power level and the secondoptical power level; (d) determining a measured extinction ratio as aratio of the detected first optical power level to the detected secondoptical power level; (e) determining an extinction ratio error as avariance between the measured extinction ratio and a predeterminedextinction ratio; and (f) using the extinction ratio error, adjustingthe modulation of the semiconductor laser to generate the optical signalhaving substantially the predetermined extinction ratio.
 20. The methodof claim 19, wherein step (f) further comprises: integrating theextinction ratio error with a plurality of previous extinction ratioerrors to form an integrated extinction ratio error; and adjusting themodulation of the semiconductor laser in response to the integratedextinction ratio error.
 21. The method of claim 19, wherein step (f)further comprises: integrating the extinction ratio error with aplurality of previous extinction ratio errors to form an integratedextinction ratio error; and adjusting the modulation of thesemiconductor laser by providing a selected current path for themodulator, the selected current path corresponding to the integratedextinction ratio error.
 22. The method of claim 19, wherein step (c)further comprises: detecting the first optical power level by sampling afirst photodetector current generated by the first optical power levelto form a first photodetector current indicator and detecting the secondoptical power level by sampling a second photodetector current generatedby the second optical power level to form a second photodetector currentindicator.
 23. The method of claim 22, wherein the sampling of the firstphotodetector current and the second photodetector current is performedby sampling corresponding voltage levels.
 24. The method of claim 22,wherein step (d) further comprises: determining the measured extinctionratio as a ratio of a first arithmetic mean of a plurality of samples ofthe first photodetector current to a second arithmetic mean of aplurality of samples of the second photodetector current.
 25. The methodof claim 22, wherein step (d) further comprises: determining themeasured extinction ratio as a ratio the first photodetector currentindicator to the second photodetector current indicator.
 26. The methodof claim 22, wherein step (c) further comprises: sampling the firstphotodetector current level when the input data signal has apredetermined number of consecutive bits having the first logical stateand sampling the second photodetector current level when the input datasignal has a predetermined number of consecutive bits having the secondlogical state.
 27. The method of claim 22, wherein step (c) furthercomprises: sampling the first photodetector current level when the inputdata signal has the first logical state for a predetermined period oftime and sampling the second photodetector current level when the inputdata signal has the second logical state for the predetermined period oftime.
 28. An apparatus comprising: a semiconductor laser capable oftransmitting an optical signal having a first optical power level inresponse to a first modulation current level and having a second opticalpower level in response to a second modulation current level, the firstoptical power level being greater than the second optical power level; amodulator coupled to the semiconductor laser, the modulator capable ofproviding the first modulation current level to the semiconductor laserwhen the input data signal has a first logical state and providing thesecond modulation current level to the semiconductor laser when theinput data signal has a second logical state, the first modulationcurrent level being greater than the second modulation current level; aphotodetector optically coupled to the semiconductor laser, thephotodetector capable of generating a first photodetector current levelin response to the first optical power level and a second photodetectorcurrent level in response to the second optical power level; a samplercoupled to the photodetector, the sampler capable of sampling the firstphotodetector current level to form a first photodetector currentindicator and sampling the second photodetector current level to form asecond photodetector current indicator; and a modulation currentcontroller coupled to the sampler and to the modulator, the modulationcurrent controller capable of determining a measured extinction ratio asa ratio of the first photodetector current indicator to the secondphotodetector current indicator; determining a variance between themeasured extinction ratio and a predetermined extinction ratio and,based on the variance, forming an extinction ratio error signal; and inresponse to the extinction ratio error signal, further capable ofadjusting the modulation current provided by the modulator to thesemiconductor laser to generate the optical signal having substantiallythe predetermined extinction ratio.
 29. The apparatus of claim 28,wherein the sampler is further capable of sampling the firstphotodetector current level to form a plurality of first photodetectorcurrent indicators and sampling the second photodetector current levelto form a plurality of second photodetector current indicators; andwherein the modulation current controller is further capable ofdetermining the measured extinction ratio as a ratio of a firstarithmetic mean of the plurality of first photodetector currentindicators to a second arithmetic mean of the plurality of secondphotodetector current indicators, and determining a variance between themeasured extinction ratio and the predetermined extinction ratio, andbased on the variance, forming the extinction ratio error signal. 30.The apparatus of claim 28, wherein the modulation current controller isfurther capable of integrating the extinction ratio error signal with aplurality of previous extinction ratio error signals to form anintegrated extinction ratio error signal; and wherein the modulationcurrent controller is further capable of adjusting the modulationcurrent by providing a selected current path for the modulator, theselected current path corresponding to the integrated extinction ratioerror signal.
 31. The apparatus of claim 28, wherein the sampler furthercomprises: a timer capable of enabling the sampling of the firstphotodetector current level when the input data signal has apredetermined number of consecutive bits having the first logical stateand enabling the sampling of the second photodetector current level whenthe input data signal has a predetermined number of consecutive bitshaving the second logical state.
 32. The apparatus of claim 28, whereinthe sampler further comprises: a timer capable of enabling the samplingof the first photodetector current level when the input data signal hasthe first logical state for a predetermined period of time and enablingthe sampling of the second photodetector current level when the inputdata signal has the second logical state for the predetermined period oftime.
 33. The apparatus of claim 28, wherein the modulation currentcontroller is further capable of providing, in response to theextinction ratio error signal, a modulation current adjustment signal tothe modulator, and wherein, in response to the modulation currentadjustment signal, the modulator is further capable of adjusting thefirst modulation current level and the second modulation current levelfor the semiconductor laser to generate the optical signal havingsubstantially the predetermined extinction ratio.
 34. The apparatus ofclaim 28, wherein the sampler further comprises: a first registercapable of storing the first photodetector current indicator when theinput data signal has a first logical state; and a second registercapable of storing the second photodetector current indicator when theinput data signal has a second logical state.